1. Field of the Invention
The present invention relates to an apparatus for determining the behaviour of powder or granular material under stress, in particular under low stress down to a minimum of zero, as for use in the design of free flowing storage hoppers.
2. Description of the Prior Art
A known device for testing the behaviour of powder or granular material is the so-called Jenike Cell. The Jenike Cell tests a cylindrical sample within rigid boundaries, assuming uniform stresses through the sample. The normal shear stresses on a horizontal plane within the sample are calculated from two external force measurements. Then, by assuming the inclination of the principal stresses to this horizontal plane, design data are obtained. It is widely recognised that neither of these assumptions can be successfully made in the Jenike Cell without elaborate precautions. The application of uniform stresses to rigid boundaries can never be assumed and at verylow stress levels the error introduced by mechanical linkages will be unacceptable.
In the field of soil mechanics, there is a class of shear testing apparatus which impose boundary stresses through flexible boundaries, see for example Arthur, Chua & Dunstan, Geotechnique Vol. 27. No. 1,1977 pages 74 to 87.
These flexible boundary apparatus approach the ideal of applying uniform and directly measured stresses when the distortions of the samples are small. However, when a very low stresses are required, the stress required to distort even the lightest flexible membrane will represent a serious error which cannot be easily quantified.